GEOLOGICAL SUCCESSION OF ANIMALS. 669 



sea-urchins and of ammonites, the older being of simpler, 

 more generalized forms, and the later with a greater 

 specialization or elaboration of the different, especially ex- 

 ternal, hard parts of the body. 



When we ascend to the Amphibians, the reptiles and the 

 mammals, we shall find that there has been an elaboration 

 or working out into great detail, of the parts most used by 

 the animal, this differentiation being more and more marked 

 as we approach the present time ; and this has been in ac- 

 cord with the building up of the continental masses, and 

 the differentiation or specialization of the surface of the 

 different continents into plains, plateaus, highlands, and 

 mountain ranges, with their different climatic features, 

 and the dividing up of the waters into mediterranean 

 seas, friths, fiords, rivers, and lakes. Thus the extinction 

 of successive faunae all over the globe has been followed by 

 the appearance of new sets of animals, each assemblage be- 

 ing adapted to the new and improved condition of things. 



Having seen that the earlier forms of life were of a sim- 

 pler form, though often combining the features of diverse 

 classes and orders of animals which appeared afterward, so 

 that Agassiz called them, in some cases, prophetic types, 

 combining as they did characters which have been trans- 

 mitted to two or more later groups, and these specially elab- 

 orated, so that such generalized or prophetic types serve as 

 points of departure from which several series of forms have 

 arisen having traced the law or principle underlying the 

 geological succession of animals, we may inquire whether 

 this has been paralleled by the development of any one of 

 the members of a group. That this is the case has been 

 proved by Hyatt, who shows that the development of the 

 individual Ammonite is paralleled by that of the geological 

 succession of the members of the order to which it belongs. 

 Stalked Crinoids were the style in Palaeozoic ages, while free 

 Crinoids are more abundant at the present day ; and we 

 have seen that in the individual development of the existing 

 Antedon, the young is stalked at first, afterward becoming 

 free. The young, bony fish has at first a cartilaginous 

 skeleton and a heterocercal tail, these being characteristics 



